Steerable beam antenna system typically consist of two basic types--reflector antennas and phased arrays. Although other antennas, such as lens antennas, are used, the reflector and phased array antenna approaches are by far the two most common. Some basic problems, however, exist with each of these antenna systems.
Reflector antennas are simple and well understood and make up the majority of high gain antenna systems. In order to steer a reflector antenna, a mechanical movement of the entire reflector is usually necessary; however, alternatives such as mechanical or electrical displacement of the feed have also been used. The speed at which the beam can be steered is limited by the mechanical limitations on accelerating the mass of the reflector or other movable parts of the antenna. The mechanical precision of the movement mechanism also limits the pointing accuracy of the antenna beam. The structure which supports the reflector surface must provide a certain precision to maximize the gain of the reflector. Surface deformation considerations also cause the structural requirements to increase significantly as the size of the antenna increases.
Phased array antennas have some advantages over reflector antennas. First, since the beam is steered electronically, the speed of beam motion is considerably faster than for a reflector antenna, especially for large regions of coverage. Pattern shaping and beam control is more straight forward and can easily be changed with the same order of speed as the beam motion. Phased arrays are usually flat and thus require considerably less depth for installation than a reflector antenna. Phased array antennas also have several disadvantages over reflector antennas. They are typically much more expensive. The efficiency of large phased array antennas is typically much lower, unless active amplifiers are distributed throughout the array, increasing the cost still further. The gain of a phased array antenna decreases as the beam is steered off broadside while a reflector antenna has constant gain if mechanically steered by motion of the entire antenna.
There have been many prior methods and apparatus for steerable beam antenna systems intended to enhance the signal, improve signal to noise ratios, measure direction and frequency and improve the resolution of such measurements and the like. Such prior methods and apparatus used with phased arrays antenna systems include, for example, those disclosed in U.S. Pat. Nos. 4,189,733; 4,544,927; and 4,652,879.